As is known, many pourable food products (e.g. fruit or vegetable juice, pasteurized or UHT (ultra-high-temperature-treated) milk, wine, etc.) are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for pourable food products known as Tetra Brik Aseptic®, which is made by folding and sealing a web of laminated packaging material.
The laminated packaging material comprises layers of fibrous material, e.g. paper, covered on both sides with heat-seal plastic material, e.g. polyethylene, and, in the case of aseptic packages for long-storage products, such as UHT milk, also comprises, on the side eventually contacting the food product in the package, a layer of oxygen-barrier material, e.g. a sheet of aluminium or EVOH, which in turn is covered with one or more layers of heat-seal plastic material.
As is known, such packages are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine itself, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed, e.g. vapourized by heating, from the surfaces of the packaging material; and the web of packaging material so sterilized is kept in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is then filled from the top with the sterilized or sterile-processed pourable food product, and is gripped at equally spaced transverse sections by two pairs of jaws. More specifically, the pairs of jaws act cyclically and successively on the tube to seal the packaging material of the tube and form a continuous strip of pillow packs connected to one another by respective transverse sealing bands.
The pillow packs are separated by cutting the relative sealing bands, and are then conveyed to a final folding station where they are folded mechanically into the finished parallelepiped shape.
In the case of aseptic packages with an aluminium layer as the barrier material, the transverse sections of the tube are normally sealed using a sealing device for inducing a parasitic electric current in the aluminium layer to melt the heat-seal plastic material locally.
More specifically, one of the jaws in each pair comprises a main body made of non-conducting material, and an inductor housed in a front seat on the main body; and the other jaw has pressure pads made of elastically yielding material, such as rubber.
The inductor is powered when the relative pair of jaws is gripped onto the tube, so as to seal a transverse section of the tube by sealing the plastic material covering.
More specifically, in addition to the inductor, the sealing device also comprises an alternating power signal generator, and a matching circuit for optimizing power transfer between the generator and inductor. The generator, in fact, supplies maximum power when the current-voltage phase angle is close to zero.
Known matching circuits are normally defined by an inductive-capacitive circuit, in which a capacitive element (normally defined by a number of parallel capacitors) is connected parallel to an inductive element (normally defined by a transformer); and the capacitance values of the capacitive element, and the inductance value of the inductive element are so selected as to produce phasing whereby the current-voltage phase angle is close to zero. Such phasing, however, is best for a predetermined electric load associated with given operating conditions (e.g. package volume, filling machine output rate and operating speed, type of inductor, etc.).
Consequently, alongside a variation in electric load caused by varying operating conditions, there is a noticeable departure from the optimum phasing condition, thus reducing power transfer to the inductor.